The number of persons suffering from circulatory organ diseases such as myocardial infarction and cerebral infarction has been increased in recent years and it is a large problem to prevent and remedy these diseases.
Hardening of arteries deeply relates to the attack of myocardial infarction and cerebral infarction. Specifically, when atheroma is formed on an arterial wall or a new cell of an artery is formed due to various factors including a high blood pressure, the artery loses elasticity and becomes hard and fragile. Moreover, when a blood vessel is obstructed at the portion where the atheroma is formed or a blood vessel tissue covering the atheroma is broken, the atheroma leaks into a blood vessel, the artery is obstructed at another portion, or the portion where the artery is hardened is broken. Thereby, these diseases are caused. Therefore, diagnosis of hardening of arteries is important for prevention or remedy of these diseases.
Whether an artery is hardened has been diagnosed by directly observing a state of the inside of a blood vessel by a blood vessel catheter. However, this diagnosis has a problem that the burden of a patient is large because it is necessary to insert a blood vessel catheter into a blood vessel. Therefore, observation by a blood vessel catheter is used for a patient whose artery is exactly hardened to specify the place where the artery is hardened but this method is not used as an inspection for health care.
Measuring a cholesterol value which is a factor of hardening of arteries or measuring a blood pressure value is an inspection which can be easily performed because the burden to a patient is small. However, these values do not directly mean a degree that an artery is hardened.
Moreover, when it is possible to administer a curative medicine of hardening of arteries to a patient whose hardening of arteries is not greatly progressed, this exhibits an effect for medical treatment of hardening of arteries. However, when the hardening of arteries is progressed, it is said that it is difficult to completely cover hardened arteries though it is possible to restrain hardening of arteries by a curative medicine.
From this reason, a diagnostic method and a diagnostic apparatus are requested which do not apply much burden to a patient and diagnose the degree of hardening of arteries before hardening of arteries progresses.
An ultrasonic diagnostic apparatus has been known so far as a medical diagnostic apparatus which does not apply much burden to a patient. By using an ultrasonic diagnostic apparatus and thereby applying ultrasonic waves from the outside of a body, it is possible to obtain shape information, motion information, or quality information on the inside of a body without providing a pain for a patient.
Particularly when performing measurement by ultrasonic waves, it is possible to obtain the elastic modulus of an object to be measured because the motion information on the object to be measured is obtained from a transmutation value. That is, it is possible to obtain the elastic modulus of a blood vessel in a living body and directly know the degree of hardening of arteries. Moreover, because measurement can be made only by touching a patient with an ultrasonic probe, the burden to the patient decreases. Therefore, by using an ultrasonic diagnostic apparatus, it is possible to accurately diagnose hardening of arteries and it is expected to apply a medical inspection for prevention to a test subject without providing burden for the test subject.
However, an ultrasonic diagnostic apparatus having been used so far does not have a very high resolution of shape information or motion information as represented by an ultrasonic diagnostic apparatus for auscultating the cardiac sound of an unborn baby. Therefore, it is impossible to obtain the elastic modulus of a contracting artery in accordance with a heart beat. For example, there are many ultrasonic diagnostic apparatuses by which transmutation measurement of an object to be measured cannot sufficiently be performed like the ultrasonic diagnostic apparatus disclosed in Japanese Patent Laid-Open No. 62-266040.
It has been possible in recent years to improve the measurement accuracy of an ultrasonic diagnostic apparatus by leaps and bounds in accordance with the progress of electronics technique. Thereby, development of an ultrasonic diagnostic apparatus for measuring micro-movement of a living-body tissue has been progressed. For example, Patent Document 2 discloses an ultrasonic diagnostic apparatus for realizing high-accuracy phase tracking by using the amplitude and phase of a detection signal and deciding the instantaneous position of an object in accordance with a constrained least square method. This apparatus can measure the micro-vibration of a tissue greatly moved by a heart beat. According to Patent Document 2, micro-vibration up to hundreds of Hz on a great vibratory transmutation movement due to a heart beat having an amplitude of 10 mm or more can be measured at a preferable reproducibility even if a heart beat is repeated approx. 10 times.
The apparatus disclosed in Japanese Patent Laid-Open No. 10-5226 can measure high frequency components up to hundreds of Hz at a preferable reproducibility and obtain the elastic characteristic of a region having a diameter of 1 to 2 mm on cardiac muscle or artery by converging an ultrasonic beam. Moreover, it is reported that there is a superior characteristic that ultrasonic oscillation of the component of every time phase can be obtained in one heart beat and frequency spectrum of the oscillation can be analyzed.
Therefore, according to the ultrasonic diagnostic apparatus of this Japanese Patent Laid-Open No. 10-5226, it is expected that it is possible to prevent a disease due to hardening of arteries by checking a progress degree of hardening of arteries with time without providing burden for a test subject in a medical inspection. Moreover, by measuring the elastic characteristic in a micro-region of an artery, it is expected that it is possible to specify a portion at which ruptured blood vessel easily occurs and remedy the portion.
Motion information on living-body tissue measured by ultrasonic waves has been visualized so far by using an image display unit such as a monitor and thereby performing two-dimensional mapping. For example, a method is widely known in which a straight line drawn by brightness-modulating an ultrasonic incoming signal in accordance with an amplitude intensity is written in an image in accordance with scanning by an ultrasonic beam to display the tomogram of a two-dimensional living body in real time. This method is referred to as B-mode method, in which a difference between living-body tissues such as internal organs is displayed in accordance with its movement and a difference between brightnesses. Therefore, for example, it is possible to observe the shape of an unborn baby or find a bilestone. In the case of a conventional ultrasonic diagnostic apparatus not having a very high resolution of shape information or motion information, the above image display coincides with the purpose of the diagnosis.
However, in the case of an ultrasonic diagnostic apparatus having a high resolution using the technique disclosed in Japanese Patent Laid-Open No. 10-5226, it is possible to measure the motion information such as the elastic information on a blood vessel wall at a resolution of 1 to 2 mm. Therefore, a state in which blood moves through a blood vessel and a blood vessel wall performs a deformation amount is measured as a motion state nearby the blood vessel. In the case of the elastic characteristic to be image-displayed, the elastic characteristic of the blood in a blood vessel is shown in addition to the elastic characteristic of a blood vessel wall to be measured. As a result, the boundary between the blood vessel wall and the blood becomes unclear and it is difficult to accurately evaluate the elastic characteristic of the blood vessel wall. Particularly, it is very difficult to find regions different from each other in elastic characteristic in the blood vessel wall and specify a portion where hardening of arteries occurs by using the above display.